Subsurface wells can be formed by a rotary drilling process. To that end, a drill bit is mounted on the end of a drill string which may be very long, e.g., several thousand feet. At the surface, a rotary drive mechanism turns the drill string and the attached drill bit at the bottom of the hole. In some cases, a downhole motor may provide the desired rotation to the drill bit. During drilling operations, a drilling fluid (“drilling mud”) can be pumped through the drill string and back up-hole by pumps located on the surface. The purpose of the drilling fluid is to, among other things, remove the subsurface formation cuttings resulting from the drilling process.
There are two basic types of boring drill bits commonly used to form the subsurface boreholes for exploration and recovery applications. The first utilizes one or more rolling cutters mounted on a bit body. There are typically several rows of cutting teeth on each cutter. When the bit body is rotated and weight is applied, the teeth on the cutters engage the formation causing the cutters to rotate. As the cutters rotate, the teeth are sequentially pushed into the formation effecting a drilling action. These bits are commonly known as rolling cutter drill bits or rock bits.
The second type of subsurface boring bit utilizes cutting elements fixed on the blades of the bit body. These bits are also rotated, and when weight is applied, the cutting elements are pushed into, and dragged through the formation. This dragging action causes formation removal by shearing. These types of drill bits are generally known as fixed cutter (FC) drill bits.
There are different FC drill bit designs for different drilling applications. For example, a high bladed steel bit (often called a fishtail bit) may be suitable for rapidly drilling through very soft soils and formations, while a polycrystalline diamond compact (PDC) bit may be used to drill through harder rock formations. For very hard and tough rock formations, an infiltrated tungsten-carbide matrix bit body is employed with diamond cutting elements. These are typically called diamond or natural diamond drill bits.
As a general rule, drill bits that are able to drill rapidly through soft formations cannot penetrate the harder formations and, similarly, drill bits that are able to drill through harder formations are not aggressive enough to economically drill through softer formations. Thus, when drilling deep wells through many different types of rock and soil, drill bits may have to be changed many times in response to wear or in response to changing soil conditions.
Common to all types of subsurface drilling bits is a means to flush the drilled formation fragments away from the cutting interface and transport them to the surface. For shallow boreholes, air is a suitable flushing fluid. However, for the deep boreholes commonly drilled for the exploration and production of hydrocarbons, the flushing fluid is typically drilling mud. Although the type of drilling fluid may vary, it typically contains abrasive elements, and it is usually pumped through nozzle orifices on the drill bit.
In rolling cutter drill bits, the primary role of drilling mud is to clean the bit and the bottom of the boreholes and transport the cuttings to the surface. In fixed cutter drill bits with PDC elements, however, the drilling mud has an added role of cooling the PDC diamonds. Diamond and other suitable forms of superhard materials are much harder than the formations being drilled, so theoretically these materials should not exhibit any wear. However, it is apparent from examination of used drill bits that the superhard cutting elements do degrade. The degradation of the superhard cutting elements can be caused by various factors.
Drilling inherently generates a significant amount of vibration as well as high forces that are necessary to efficiently fail rock. However, excessively high loading leads to premature cutter failure and expensive trips/repairs. There have been many different attempts to address these problems. For example, one conventional method to provide erosion resistance is to apply welded hardmetal in thick layers to the surface of the blades of a steel body drill bit. Unfortunately, welded hardmetal can crack as the blades of the PDC drill bit bend in response to the drilling loads. Once a crack starts, the impinging drilling fluid quickly erodes the exposed, soft underlying steel. Conventional bit designs are described in U.S. Pat. Nos. 7,318,492, 7,237,628, 7,036,613, 6,878,447, 6,861,137, 6,861,098, 6,797,326, and 6,739,214 (all assigned to the present assignee and each entirely incorporated herein by reference).
There remains a need for improved bit designs to provide increased resiliency and structure durability.